1. FIELD OF THE INVENTION
This invention relates generally to gas-purification or "scrubbing" equipment used in wastewater-treatment plants to clean so-called "digester gas," and relates more particularly to novel procedures and apparatus for safely and economically regenerating the chemically active medium (hydrated iron-oxide "sponge") used in such equipment.
2. PRIOR ART
"Digester gas" is a gaseous mixture produced by the anaerobic digestion of carbonaceous waste, particularly in wastewater treatment. Digester gas contains primarily methane gas--together with carbon-dioxide gas, a small quantity of hydrogen-sulfide gas, and in general trace amounts of various other gases.
The methane in the digester gas is valued as a fuel. The hydrogen sulfide, however, if left in the gaseous mixture would create a risk of pipeline and burner corrosion as well as other problems. Accordingly it is desirable to "scrub" the hydrogen sulfide from the mixture.
A permeable bed of wood chips coated with hydrated ferric oxide--often called "iron-oxide sponge"--is 8#commonly used as a medium to purify digester gas by removing the harmful hydrogen sulfide. This is accomplished by passing the digester gas downward through a vessel that contains the iron-oxide sponge, as shown in FIG. 2.
As illustrated, the vessel 11 has an inlet port 12 near the top for entry of the digester gas 21 to be purified, and an outlet port 13 near the bottom for exit of the "scrubbed" gas 25. A perforated platform 14 near the bottom of the vessel 11, just above the scrubbed-gas outlet 13, supports the purifying medium 31.
Thus the passage of the digester gas 21, 25 through the medium is downward as at 23, for collection as at 24 below the platform 14. This direction of gas flow is preferred, and to the best of my knowledge is used universally. This preference is not arbitrary, but rather arises from the following important rationale.
Upward gas flow would cause "channeling"--disruption and rearrangement of the medium 31 by the lifting effect of the upward stream, and then effective confinement of the flow to a relatively small number of well-defined, enlarged paths or channels in the medium. Channeling reduces drastically the total surface area to which each small volume of digester gas is exposed, and correspondingly reduces the effectiveness of the purification process.
Downward flow 21-23-24-25, by contrast, actually tends to gently compact the medium 31. Such compaction enhances the diffusion of gas flow throughout the medium 31 and with it the effectiveness of scrubbing. Consequently, piping the scrubber vessel 11 for downward flow is an important teaching of the prior art. #The top of the vessel 11 is provided with a small spray head 15, pipe 16 and valve 17 from a water supply 18--to facilitate application of a light misting spray 32 to moisten the medium 31 in preparation for use. It is well known that iron-oxide sponge 31 does not effectively combine with hydrogen-sulfide gas in the desired fashion unless moisture is present.
Once the scrubbing operation is begun, the sprayer valve 17 usually can be turned off, because enough moisture is carried into the vessel 11 with the digester gas 21 (from the wastewater) to ensure effective scrubbing. In fact, excess moisture is sometimes introduced in this way, and an additional very small amount of water is produced by the scrubbing reaction itself; hence a small drain 41 and drain valve 42 are generally provided at the bottom of the tank 11--only for removing the small amounts of excess moisture.
Even when it is used, the spray head 15 has very limited flow capacity: it is designed only to moisten. Accordingly, as an example, in a tank of a meter-and-a-half diameter and equal height the sprayer 15 and valve 17 are typically capable only of perhaps ten to fifteen milliliters per second--that is to say, on the order of one sixtieth of the tank capacity in an hour.
The digester gas 21, 23 is cleansed of hydrogen sulfide as the latter combines with the hydrated ferric oxide in the medium 31 to form a solid, ferric sulfide --that remains in the medium 31--and a very small amount of liquid, water. As previously mentioned, the water 33 if in sufficient quantity can trickle out through the drain 41 and valve 42. The remaining gas 24, 25 passes through and leaves the bed 31 and vessel 11 at the bottom port 13, for subsequent use as fuel.
This reaction eventually converts substantially all of the iron oxide to ferric sulfide, terminating the purifying ability of the medium 31. That is to say, when all of the available hydrated ferric oxide has reacted to form ferric sulfide, no further hydrogen sulfide is removed from the gas stream 21-23-24-25.
The scrubber 11 then must be removed from operation, and the spent medium 31 either replaced or regenerated. These alternatives are the focus of my invention.
It is possible to regenerate the iron-oxide sponge 31 in the vessel 11 by sweeping the vessel with an inert gas and a carefully controlled small amount of oxygen. The chemical environment should be neutral or alkaline.
In principle this procedure, which is well-known in the wastewater industry and in fact may appear in instructions furnished by some scrubber manufacturers, reverses the above-described reaction to reform hydrated ferric oxide. The reaction releases elemental sulfur, which remains in the medium as a solid.
Much or all of the solid sulfur remains in place on the medium, and tends to mask the subsequent operation of the medium. As a result, even under best theoretical conditions the regeneration can be performed only two or three times before the accumulation of sulfur renders the medium unusable permanently. The only option is then replacement of the medium. In practice, however, even this theoretical optimum is not realized, for the following reasons.
The regenerating process liberates heat and can cause spontaneous combustion. Considering the size of the equipment involved it will be understood that such a procedure may be dangerous in the extreme. The process therefore requires constant and vigilant monitoring over a twenty-four-hour period or longer, and is unforgiving of human error.
Consequently many or all operators elect to forego the process entirely. Such operators either use an even more time-consuming and costly purification procedure or replace the iron-oxide sponge with new material.
The more time-consuming procedure is to remove the spent medium entirely from the vessel, spread the medium out loosely on ground or pavement to a depth of a few inches keep it moist, and wait a few days for natural air circulation through the medium to effect regeneration.
That alternative procedure is costly and time consuming, but at least safe. The same may be said of complete replacement with new iron-oxide sponge.
In order to better appreciate my invention it will be helpful to have a somewhat more detailed understanding of the prior-art in-tank regeneration process--although, as just explained, it is seldom or never used.
In the prior-art process the flow of digester gas is halted, and as shown in FIG. 3 the digester-gas mixture is purged from the vessel with an inert gas such as carbon dioxide or nitrogen. A source 43 of such gas is connected to a valve 44 and suitable piping 45, 46 to an inlet 47 near the top of the vessel 11. The inert gas 26 is passed as at 27 through the bed 31 from above, sweeping digester gas out of the bed 31 for collection as at 28 and exit through a purge port 48 near the bottom.
When substantially all the digester gas is removed, the valve 44 is closed, and a pump 51 is actuated to recirculate the inert gas through the bed--still from above--as shown in FIG. 4. The inert gas is drawn from the bottom of the vessel 11 through a small outlet 53 and through a pipe 54 into the pump 51. From the pump the gas passes through pipes 52, 46 to a gas inlet 47 and thereby back into the tank 11.
Then a valve 56 is opened to admit a small stream of air from ambient or from any other appropriate source 57 through piping 58 into the pump 51 or otherwise into the recirculating gas stream. This admixture forms a regenerating gas 29, also passed through the medium downward--again, to avoid channeling during the regeneration process itself, or disruption of the medium 31 that could produce channeling later when the scrubber is returned to service.
Because the chemical reaction of oxygen and ferric sulfide to reform hydrated ferric oxide and elemental sulfur is exothermic, the temperature of the gas stream will rise. The temperature rise must be restricted by throttling back the air-injection valve 56. A thermometer 59 is provided for monitoring the temperature, which should be maintained below 120 .degree. F.
In the event that the temperature rises out of control, the water of hydration is removed from the hydrated ferric oxide above 150.degree. F. At 212.degree. F. steam is produced, driving off moisture, and at approximately 700.degree. F. the kindling temperature of the wood-chip substrate is reached.
In such a situation, as air is present, burning results. This juncture may be reached abruptly and--with due regard to the character of combustion in a closed vessel it is not unrealistic to say--explosively.
If the operator succeeds in avoiding such exigencies, continuously providing the maximum amount of air consistent with restraining the temperature, the regeneration procedure can be expected to require up to thirty-six hours. The process may be suspended from time to time by repurging the vessel 11 with inert gas, but of course this extends the period in which the scrubber is out of service.
Regeneration is complete when the temperature of the recirculating gas remains ambient and unaffected by the continued admission of air to the stream. As already noted, however, this procedure is virtually an abstraction since essentially all operators elect to remove the scrubber medium for regeneration, or simply to replace it.
From the foregoing it will be clear that prior-art processes and equipment fail to provide convenient, fast, economical and safe regeneration.